Threatening to capture the pawn on b3, followed by a mating attack.
19. d3d2  19... b4b3 Black's attack appears to arrive quickly, but the situation is not yet so desperate for White. 20. c2b3  20... a4b3  21. a2b3 [%c_effect b3;square;b3;type;Mistake;persistent;true] A serious error, mortally weakening the queenside.